A brushless single-phase direct current (DC) motor typically includes a rotor containing one or more permanent magnets and a stator containing a winding. A current is applied to the stator winding to produce a magnetic field, and the rotor is induced to rotate due to opposition between the respective rotor and the stator magnetic fields. The direction of current flow in the stator winding must be reversed twice for each revolution of a two-pole rotor in order to provide successive field opposition as the rotor rotates. The act of changing the direction of the flow of current in the stator winding is referred to as commutation. The mechanical power provided by a motor is dependent on when the commutation is performed relative to a back electromotive force (BEMF) that is induced in the stator winding by the magnetic field of the rotating rotor.
A sensor such as a Hall effect sensor can be used to identify the angular position of the rotor, but this technique requires an additional electronic controller to effectively predict an ideal commutation time. Moreover a Hall effect sensor adds cost to the product. Other techniques for determining commutation time, such as the use of extra stator windings to directly sense the BEMF, and still other techniques that attempt to monitor the BEMF by detecting variation in stator current supplied by a motor controller, also do not predict the ideal commutation time and/or add to the product cost. Motor efficiency can be improved if commutation time can be better controlled.
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